Knowledge

Stainless steel 304, known for its corrosion resistance, workability, and economic efficiency, is hailed as the "general-purpose stainless steel" and is widely used in food equipment, architectural decoration, medical devices, and other fields. However, in practical applications, users often find that 304 stainless steel still shows rust spots, pitting, or even uniform corrosion under certain conditions. Behind this paradox lies a complex interplay of material science, environmental factors, and surface treatment techniques. This article will analyze the issue from three aspects: the composition characteristics of 304 stainless steel, the causes of corrosion, and the necessity of passivation processes.

I. The "Corrosion Resistance" of 304 Stainless Steel: The Fragile Balance of the Passivation Film

The core corrosion resistance mechanism of 304 stainless steel (06Cr19Ni10) lies in the chromium oxide passivation film formed on its surface. When the chromium content is ≥10.5%, the steel will spontaneously form a Cr₂O₃ film about 2-5 nanometers thick in an oxidizing environment (such as air). This film reduces the corrosion rate to less than one-thousandth of that of ordinary carbon steel through physical barrier, electrochemical protection, and self-repairing capabilities.

However, this protective system has inherent limitations:

1. Composition Dependency: The stability of the passivation film is highly dependent on the continuous supply of chromium. If the surface chromium is consumed (such as through long-term contact with acidic media), the film will gradually fail.

2. Environmental Sensitivity: In environments containing chloride ions (Cl⁻), sulfides, or high temperatures and humidity, the passivation film may be damaged, accelerating local corrosion.

3. Processing Damage: Processes such as cutting and welding can damage the original passivation film, and mechanical damage may penetrate into the base material, exceeding the self-repairing capacity of the film.

II. Five Major Causes of Rusting in 304 Stainless Steel

1. Chloride Ion Corrosion: The Nemesis of Passivation Film

Chloride ions (such as in seawater, table salt, and industrial brine) are the main culprits of 304 stainless steel corrosion. Their mechanisms of action include:

- Penetration effect: The small radius of Cl⁻ (0.181 nm) enables it to penetrate through membrane defects or grain boundaries, forming soluble complexes with chromium (such as CrCl₃), leading to local dissolution of the film.

- Electrochemical corrosion: Cl⁻ accumulates at defects, forming "active-passive" microcells that accelerate the expansion of pitting corrosion.

- Case: 304 stainless steel railings used in coastal areas may show obvious pitting corrosion within three years if not passivated, while similar products in inland dry environments can last for over ten years.

2. Processing Defects: Hidden Corrosion Pathways

Mechanical processing (such as cutting, stamping, and welding) can damage the original passivation film and introduce the following risks:

- Heat-affected zone (HAZ) sensitization: During welding, in the 450-850°C range, carbon combines with chromium to form chromium carbide (Cr₂₃C₆), reducing the chromium content near the grain boundaries to below the critical corrosion-resistant value (10.5%), creating "chromium-depleted zones".

- Surface roughness: When the surface roughness (Ra) after processing exceeds 0.8 μm, corrosion media can accumulate in the grooves, forming closed cells.

- Case: A stainless steel mixing tank in a food processing plant developed through-wall corrosion at the weld seams after two years of use due to the lack of acid washing and passivation after welding.

3. Imbalance of medium pH: A fatal blow from acidic environments

304 stainless steel performs best in a neutral environment with pH ranging from 6 to 10. However, in strong acidic or alkaline conditions:

- Acidic medium: H⁺ reacts with Cr₂O₃ to form Cr³⁺, destroying the oxide film structure. For instance, a 5% or higher concentration of hydrochloric acid solution can completely dissolve the passive film within a few hours.

- Alkaline medium: High concentration of OH⁻ promotes the dissolution of Fe, forming Fe(OH)₃ precipitate, which makes the oxide film loose and porous.

- Case: A chemical enterprise used 304 stainless steel to store dilute sulfuric acid without considering the acidity of the medium. After three months, uniform corrosion perforations appeared on the inner wall of the equipment.

4. Temperature effect: High temperature accelerates oxide film failure

An increase in temperature significantly reduces the stability of the oxide film:

- Dynamic corrosion: In an environment above 80°C, the diffusion rate of Cl⁻ increases tenfold, and pitting corrosion sensitivity surges.

- Oxide film decomposition: Beyond the critical temperature (approximately 300°C), Cr₂O₃ converts to volatile CrO₃, causing permanent damage to the oxide film.

- Case: The 304 stainless steel condenser in a coastal power plant, which had been operating in 60°C seawater for five years without passivation treatment, saw a 30% reduction in the thickness of the tube walls.

5. Surface contamination: Catalytic corrosion by organic matter

Dust, oil, fingerprints, and other surface contaminants can form "micro-electrolytic cells":

- Organic matter decomposition: Microorganisms reproduce in the contaminants and secrete organic acids (such as acetic acid and lactic acid), lowering the local pH value.

- Differential aeration corrosion: The areas covered by contaminants and the exposed areas form an oxygen concentration difference, accelerating local corrosion.

- Case: The stainless steel operating table in a hospital developed yellowish-brown rust spots due to long-term contact with disinfectant residues and lack of timely cleaning and passivation.

III. Passivation Process: A Key Technology for Rebuilding the Anti-corrosion Defense Line

The passivation process artificially constructs a denser and more stable passivation film on the surface of 304 stainless steel through chemical or electrochemical methods. Its necessity is reflected in the following aspects:

1. Repairing processing damage and rebuilding the protective layer

- Pickling and passivation: By using the pickling and passivation process, the oxide scale and chromium-depleted layer produced by welding can be dissolved simultaneously, and a Cr₂O₃ film can be regenerated on the surface. Experiments show that after pickling and passivation, the pitting potential of 304 stainless steel in a 3.5% NaCl solution increases from 0.2V to 0.6V (vs. SCE).

- Electrolytic polishing: By electrochemically dissolving the surface micro-protrusions, the roughness can be reduced to Ra < 0.1μm, reducing the retention areas for corrosive media.

2. Enhancing the corrosion resistance of the film layer

- Film layer thickening: By extending the passivation time or increasing the solution concentration, the film layer thickness can be increased from 2-5nm to 10-20nm, prolonging the penetration time of Cl⁻.

IV. Key Points for the Implementation of Passivation Process

1. Pretreatment: Thoroughly remove oil stains, oxide scale and welding spatter to ensure surface cleanliness reaches Sa2.5 grade.

2. Passivation solution ratio: Select passivation solution and process based on environmental requirements.

3. Time control: The passivation time is usually 15-30 minutes. A time that is too short will result in incomplete film formation, while a time that is too long may cause over-corrosion.

4. Post-treatment: After passivation, rinse with deionized water and dry immediately to avoid water stains causing electrochemical corrosion.

The "rusting" of 304 stainless steel is not a material failure but a result of the combined effect of the environment and processing. The passivation process, through artificial intervention, re-establishes the chemical balance on the material's surface, transforming the passive reliance on spontaneous passivation into the active construction of a protective system. Under harsh working conditions such as chloride ion contamination, high temperature and humidity, or mechanical processing, passivation treatment is not only a means to extend service life but also a necessary measure to ensure the safe operation of equipment.

Vigor has more than 20 years experience and the professional team in Casting and Die-forging process and the post treatment. If you have any question or products need to do, please feel free to contact us at info@castings-forging.com